Transposition of yeast Tyl elements closely resembles aspects of retroviral replication. In these retrotransposons, a virus-like particle (VLP) is an obligate intermediate in transposition, just as a similar nucleoprotein complex is an intermediate in the integration of retroviruses. Reverse transcription of a genomic RNA molecule takes place in this capsid-like structure in both cases. The exact roles of Tyl-encoded functions, and, to a lesser extent, host functions, in these processes are only beginning to be understood. We seek to understand the detailed mechanisms of reverse transcription and integration used by Tyl in the transposition process. We plan to tackle this problem using a combined genetic, biochemical, and molecular biological approach. Specific aims are: 1) To identify host functions required for Tyl transposition and determine their role; 2) Tp characterize the host TYTl gene, which is required for efficient Tyl transposition, characterize TYTl gene product(s), and determine its role in transposition; 3) To determine the mechanisms of (-) strand priming in Tyl reverse transcription; 4) To determine the sequences required in cis at the termini of Tyl for transposition in vitro; 5) To develop and exploit a new assay for transposition in vitro and 6) To examine the factors influencing target specificity of Tyl transposition in vivo and in vitro.